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Abstract: Sea area is an important area of oil and gas exploration in China. It has been found that China’s sea area mainly consists of coal type oil and gas, and the exploration of coal-bearing series source rocks has become an important part of oil and gas exploration there. Through years of comprehensive geological research in China’s sea area, it has been revealed that it has undergone multiple occurrences of tectonic opening and closing movements in varying degrees in the Paleogene, forming 26 Cenozoic sedimentary basins of various types, such as active continental margin, passive continental margin, transitional continental margin and drift rift basins. In the present study, it is observed for the first time that coal type source rocks are mainly developed in 14 continental margin basins in China’s sea area, revealing that a very large C-shaped coal-bearing basin group developed there in the Cenozoic. Next, based on the coupling analysis of paleoclimate, paleobotany, paleogeography and paleostructure, it is observed that there are five coal-forming periods in China’s sea area, namely the Paleocene, Eocene, early Oligocene, late Oligocene and Miocene-Pliocene, and the coal-forming age is gradually new from north to south. It is also found that the coal seams in the sea area are mainly developed in three coal-forming environments in Cenozoic, namely delta, fan delta and tidal flat-lagoon. The coal seams developed in different environments are characterized by thin thickness, many layers and poor stability. However, the coal-bearing series source rocks in China’s sea area have a wide distribution range, very high thickness and large amount, thus forming a material basis for the formation of rich coal type oil and gas.
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Key words:
- coal-bearing basin belt /
- coal-forming period /
- coal-forming sedimentary environment /
- China’s sea area
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Figure 1. Tectonic evolution map of the SCS and its adjacent areas (from Zhang et al., 2015a).
Figure 2. Coal seam distribution and coal-forming sedimentary facies in China’s sea. ①: Xuhu Delta; ②: Baiyun Delta; ③: Liwan Delta; ④: Honghe Delta; ⑤: fan delta belt in the northern margin of the northern depression zone in Qiongdongnan Basin; ⑥: fan delta belt in northern margin of central depression zone in Qiongdongnan Basin; ⑦: fan delta belt in northern margin of central depression zone in Qiongdongnan Basin; ⑧: Wan’an Delta; ⑨: Lupal Delta; ⑩: Rajan Delta; ⑪: Beikang Delta; ⑫: Balan Delta; ⑬: Merigan Delta; ⑭: Champion Delta; ⑮: North Palawan Delta.
Figure 6. Sedimentary facies of the Pinghu Formation in the Xihu Sag, East China Sea Basin1) (Shen, 2018; Jiang et al., 2020).
Table 1. Geological characteristics of coal formation in five typical coal-forming periods
Content period Paleocene Eocene Early Oligocene Late Oligocene Miocene‒Pliocene Typical basin East China Sea Basin, Taixi Basin Xihu Depression Nansha drift basin Zhujiang River Mouth Basin Qiongdongnan Basin, Wan’an Basin, Zengmu Basin Yinggehai Basin, Zengmu Basin, Brunei-Sabah Basin Coal-bearing strata Lingfeng Fm., Mingyuefeng Fm., Mushan Fm., Daliao Fm., Shidi Fm., Nangang Fm., Nanzhuang Fm. Pinghu Fm. Nanwei Group Enping Fm., Yacheng Fm. Xiwei Group Zengmu Fm. Lidi Fm. Stage IVA Paleoclimate temperate climate, early dry heat, late warm humid‒dry heat‒warm humid alternating subtropical climate, warm, vertical with warm wet‒dry heat‒warm wet alternating subtropical climate, hot and humid tropical climate, hot and humid with gradual cooling subtropical climate, hot and humid Paleobotany terrestrial higher plants dominated terrestrial higher plants dominated; temperate type, wet‒hot type plants mixed mainly subtropical evergreen deciduous broad-leaved forest higher plants dominated by mangroves higher plants dominated by mangroves Paleostructure continental margin rifting stage, fault basins developed, with the strongest rifting in the South Yellow Sea and the western East China Sea rifting was strengthened, and
the fault depression developed very well, forming the Pinghu fault depression in the East China SeaEnping fault depression was formed in the north of Zhujiang River Mouth Basin, uplift and denudation in the west sag of the Donghai Basin, and subsidence in the east formed continental fault depression new South China Sea began to expand and the Paleo South China Sea began to shrink Yellow Sea and East China Sea entered a period of regional subsidence, the expansion of the new South China Sea reached its peak, and the ancient South
China Sea continued to shrinkPaleogeography marine facies, marine-continental transition facies marine facies, marine-continental transition facies marine-continental transition facies, Marine facies marine facies, marine-continental transition facies, continental facies marine facies, marine-continental transition facies Favorable coal-forming zone delta plain, coastal plain coastal plain, tidal
flat swamp, lagoon
siltation swampcoastal swamp,
river-lake swampdelta plain, bay, tidal flat swamp coastal plain, delta plain -
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